157
The process that allows the formation of sperm from germline stem cells is called
spermatogenesis. In most mammals, this production continues throughout life, but in
others, spermatogenesis is seasonal. This process takes place within the seminiferous
tubules. In the first phase of spermatogenesis, differentiating germ cells progress
between Sertoli cells, from the base to the lumen of the tubules. Three major steps can
be identified: the proliferation of stem cells, spermatogonia, by successive mitosis
allows their renewal; meiosis which allows, from spermatogonia, to obtain spermatocytes then round spermatids; and finally, the differentiation of spermatids into highly
differentiated cells such as spermatozoa. This last step is called spermiogenesis.
Eventually, the spermatozoa are released into the lumen by a process called spermiation. Spermatozoa are moved along a series of ducts in the testis toward a structure
called the epididymis for the final steps of post-testicular maturation.
Spermatogonia Spermatogonia are dividing by mitosis and constitute a pool of
cells for spermatogenesis. Spermatogonia are divided into two classes according to
nuclear morphological criteria, i.e., type A and type B spermatogonia. In order for
spermatogenesis to be continuous, it is necessary that spermatogonia not only enter
into meiosis, but also self-renew to maintain a constant stock of stem cells within
the seminiferous tubules.
Spermatocytes Meiosis consists of two successive cell divisions and allows from a
stem cell with 2n chromosomes, the spermatogonia, to obtain n chromosome cells, the
spermatids. During the first meiotic division, primary spermatocytes will undergo
numerous chromosomal rearrangements allowing a genetic mixing. At the end of the
second meiotic division, the secondary spermatozoa will generate haploid cells, the
spermatids.
Spermatids and Spermiogenesis The differentiation of spermatids into spermatozoa is called spermiogenesis. This process includes many morphological changes.
Spermatids resulting from meiosis will grow longer, lose a lot of cytoplasm and
acquire a locomotion structure, the flagella. The loss of cytoplasm is caused by the
formation of residual bodies that will be phagocytosed by Sertoli cells at the moment
when older spermatids are released into the seminiferous tubule lumen. Other
changes also occur in the nucleus of spermatids. Chromatin condenses resulting in
a complete repression of gene transcription.
10.2.1 The Cycle of the Seminiferous Epithelium
Depending on the development of the spermatid acrosome and the shape of the
nuclei, Leblond & Clermont historically defined 14 stages or associations of germ
cells whose composition is constant in the rat with spermiogenesis broken down
further into 19 differentiation steps providing a striking and unique example of cell
differentiation involving acrosome formation, nuclear condensation and flagellar
biogenesis [8].
10 An Overview of Male Reproductive Toxicants: Facts and Opinions
The process that allows the formation of sperm from germline stem cells is called
spermatogenesis. In most mammals, this production continues throughout life, but in
others, spermatogenesis is seasonal. This process takes place within the seminiferous
tubules. In the first phase of spermatogenesis, differentiating germ cells progress
between Sertoli cells, from the base to the lumen of the tubules. Three major steps can
be identified: the proliferation of stem cells, spermatogonia, by successive mitosis
allows their renewal; meiosis which allows, from spermatogonia, to obtain spermatocytes then round spermatids; and finally, the differentiation of spermatids into highly
differentiated cells such as spermatozoa. This last step is called spermiogenesis.
Eventually, the spermatozoa are released into the lumen by a process called spermiation. Spermatozoa are moved along a series of ducts in the testis toward a structure
called the epididymis for the final steps of post-testicular maturation.
Spermatogonia Spermatogonia are dividing by mitosis and constitute a pool of
cells for spermatogenesis. Spermatogonia are divided into two classes according to
nuclear morphological criteria, i.e., type A and type B spermatogonia. In order for
spermatogenesis to be continuous, it is necessary that spermatogonia not only enter
into meiosis, but also self-renew to maintain a constant stock of stem cells within
the seminiferous tubules.
Spermatocytes Meiosis consists of two successive cell divisions and allows from a
stem cell with 2n chromosomes, the spermatogonia, to obtain n chromosome cells, the
spermatids. During the first meiotic division, primary spermatocytes will undergo
numerous chromosomal rearrangements allowing a genetic mixing. At the end of the
second meiotic division, the secondary spermatozoa will generate haploid cells, the
spermatids.
Spermatids and Spermiogenesis The differentiation of spermatids into spermatozoa is called spermiogenesis. This process includes many morphological changes.
Spermatids resulting from meiosis will grow longer, lose a lot of cytoplasm and
acquire a locomotion structure, the flagella. The loss of cytoplasm is caused by the
formation of residual bodies that will be phagocytosed by Sertoli cells at the moment
when older spermatids are released into the seminiferous tubule lumen. Other
changes also occur in the nucleus of spermatids. Chromatin condenses resulting in
a complete repression of gene transcription.
10.2.1 The Cycle of the Seminiferous Epithelium
Depending on the development of the spermatid acrosome and the shape of the
nuclei, Leblond & Clermont historically defined 14 stages or associations of germ
cells whose composition is constant in the rat with spermiogenesis broken down
further into 19 differentiation steps providing a striking and unique example of cell
differentiation involving acrosome formation, nuclear condensation and flagellar
biogenesis [8].
10 An Overview of Male Reproductive Toxicants: Facts and Opinions